1. Field of the Invention
The present invention generally relates to a method and apparatus for efficiently using system resources in a mobile communication system. More particularly, the present invention relates to a method and apparatus for estimating the velocity of a Mobile Station (MS) without adding overhead.
2. Description of the Related Art
In order to support large-capacity multimedia services within limited frequency and channel resources, a future-generation wireless communication system should allocate system resources based on accurate channel information. Velocity is a very significant factor that determines channel information about a time-variant channel such as a mobile communication channel in the wireless communication system. Since the velocity is information indicating the channel status of a user, it is significant to efficient utilization of system resources.
In real applications, the velocity information of MSs can increase system performance. For example, when it is used in adaptive transmission/reception technology, the velocity information allows for efficient channel estimation at an MS receiver and enables a transmitter to control a modulation and coding scheme or an interleaving method for the MS according to the channel status of the MS. If the velocity information is used in a network-implemented algorithm, an accurate decision can be made on a handover and system resource allocation can be efficiently managed.
The velocity information of the MS can also be utilized in selecting a signal transmission scheme in a BS with a plurality of antennas. For the BS, beamforming, Spatial Division Multiple Access (SDMA), transmit diversity, spatial multiplexing, etc. are available for signal transmission. Among these transmission schemes, beamforming offers a better performance for a low-speed MS, and a Multiple Input Multiple Output (MIMO) scheme such as transmit diversity or spatial multiplexing outperforms the other transmission schemes for a high-speed MS. In this manner, the system capacity can be increased by selecting an appropriate transmission scheme between MIMO and beamforming based on the velocity information of the MS, by the BS.
Conventionally, a mobile velocity estimation technique was proposed to increase the accuracy of channel estimation in the MS receiver. The MS receives a predetermined pilot signal or a constant envelope signal through the receiver and estimates a maximum Doppler frequency based on a variation in the amplitude of the received signal. COVariance (COV) and Level Crossing Rate (LCR) are major examples of this mobile velocity estimation scheme. COV measures the covariance between the amplitudes of two signals spaced by a predetermined time interval and estimates a maximum Doppler frequency using the covariance. LCR measures a rate at which the amplitude of a signal crosses a threshold level for a predetermined time period and estimates a maximum Doppler frequency using the rate.
These conventional velocity estimation methods that estimate a maximum Doppler frequency according to the velocity of an MS are based on the condition that the transmitter of a BS or the MS transmits a pilot signal or a constant envelope signal. If the purpose of the velocity estimation is to increase channel estimation accuracy, the condition is easily fulfilled. However, when the BS needs to estimate the velocity of the MS on the downlink, the MS should transmit an additional uplink signal, i.e., when data is transmitted only on the downlink data transmission, without an uplink data transmission, it is impossible to estimate the velocity of the MS.
Even if the BS uses an uplink control signal that is always transmitted irrespective of the uplink data transmission, it has difficulty in estimating the velocity because the envelope size of the uplink control signal is not constant due to power control. In this case, it can be contemplated that the MS estimates its velocity using a downlink signal and feeds back the velocity, or the MS transmits an additional signal by which the BS can estimate the velocity of the MS. However, these methods have the shortcomings of increased overhead and thus decreased system capacity.